For markup, it uses React just as a simple template engine. The code is converted to static HTML using the renderToStaticMarkup API, so there is no state, context, virtual DOM, nothing, just the RAW HTML. Why? This page is pretty simple and it doesn't need all the code needed to make React work on the client, and could be served as a static page (no SSR then). The resulting HTML is minified using html-minifier.
For styling it uses sass, compiled with sass.compile and embedded in the HTML wrapped by a style tag (one request less to the client).
For client js it uses simple vanilla JS, also minified by html-minifier and embedded with a script tag (again, one request less).